Redis fixes

Closes #117 and #114

See merge request !140

The redigomock package is GPLv2. Because I am unsure if this is
compatible with our own license and because we will have the GitLab
9.0 release soon I am adding this hack to keep redigomock in our test
suite but out of our sources.

Also: https://gitlab.com/gitlab-org/gitlab-workhorse/issues/114

Bug-fix for redis connection

See merge request !137

Fix for internal/redis.TestWatchKey{AlreadyChanged,MassiveParallel}

Closes #111

See merge request !136

Track errTooManyRequests in prometheus

Closes #106

See merge request !127

Don't log errors to Sentry.
Reduce helpers usage in favor of simple http error

Do not follow redirects when doing api calls.

Closes #107

See merge request !134

+ small code cleanup

Make redirects work in go < 1.7

Support v4 CI API

Closes #110

See merge request !133

Version 1.4.0

See merge request !139

Mark Redis experimental while we sort out bugs

See merge request !138

Temporary measure so we can cut a release from master while some Redis
integration issues are still being sorted out.

https://gitlab.com/gitlab-org/gitlab-workhorse/issues/113

https://gitlab.com/gitlab-org/gitlab-workhorse/merge_requests/136

Add metrics for queueing mechanism

See merge request !122

Add apiCiLongPolling option to use a new redis queueing mechanism

See merge request !128

Rename gitlab_workhorse_builds_register_handler_hits to gitlab_workhorse_builds_register_handler_requests